Recent investigation shows that, the apparent consumption of diesel fuel in China has already mounted up to about 167 million tons, which leads to frequent occurrence of short supply of diesel fuel (the domestic demand ratio of diesel fuel to petrol is about 2.5:1, but the production ratio thereof is about 2.3:1). Besides the reasons of unreasonable pricing of different types of oil products, and slow price linkage mechanism of domestic petroleum products with international crude oil, the fundamental reason is the constraints of resource shortage. Traditionally, diesel fuel is made from petroleum based feedstock, and the resource endowment of China characterized in relatively “rich in coal, poor in oil, and lack in gas” leads to increasingly prominent contradiction between petroleum supply and relatively fast sustainable development of economics and society. Since China became a net importer of petroleum in 1993, the import volume increases fast and constantly, and the foreign-trade dependence already exceeded 56% after 2011, it has a severe impact on national strategic security of energy.
Furthermore, the worsening crude oil quality leads to continuous scale expansion of domestic catalytic processing of heavy oil and increasing percentage of diesel fuel produced by catalytic processing, which results in gradual decline of the cetane number (CN value) of diesel fuel products and significant increase of noxious substance discharged after combustion, therefore, the urgent problem to be solved is to increase the CN value of diesel fuel.
The tail gas discharged by a diesel engine contains, besides CO, CO2 and NOx, a large amount of noxious substance such as unburned hydrocarbon compounds (HC) and particulate matter (PM), which is one of the main sources of PM2.5 contamination in urban air. International Agency for Research on Cancer (IARC) affiliated with World Health Organization (WHO) declared in June, 2012 the decision to elevate the cancer hazard ranking of diesel engine tail gas, from “possibly carcinogenic” classified in 1988 to “definitely carcinogenic”. As scientific research advances, now there is enough evidence to prove that diesel engine tail gas is one of the reasons that cause people to suffer from lung cancer. Furthermore, there is also limited evidence indicating that, inhaling diesel engine tail gas is relevant to suffering from bladder cancer. IARC hopes that this reclassification can provide reference for national governments and other decision makers, so as to actuate them to establish more strict discharge standards of diesel engine tail gas. This significant decision undoubtedly puts forward more rigorous requirements of diesel fuel quality.
Reducing the content of noxious substance such as sulfur, nitrogen and aromatic hydrocarbon in fuels by petroleum refining process such as hydrofining is an effective technical route to improve fuel quality, but has very demanding requirements of hydrogenation catalyst and reaction process, with relatively high processing cost. Internationally, many scientific research institutes are carrying out research and development on production technologies of oxygen-containing blending components for petrol and diesel fuel, especially those diesel fuel blending components with high oxygen and high cetane number, and this has recently become a research hotspot in the technical field of new energy.
Polyoxymethylene dimethyl ethers (also known as polymethoxymethylal, dimethyl-polyformal, with the general formula of CH3(OCH2)nOCH3, abbreviated as DMMn, n=8), which is a yellow liquid with a high boiling point, an average cetane number reaching above 76 and increasing dramatically with the increase of its polymerization degree, an average oxygen content of 47%-50%, a flashing point of about 65.5° C., and a boiling point of about 160-280° C., is a clean diesel fuel blending component with a high cetane number. When blending into ordinary diesel by a certain percentage (e.g., 15v %), it can significantly increase oxygen content of diesel fuel products, so as to promote sufficient combustion of diesel fuel and to sharply reduce the discharge of combustion-generated pollutants such as NOx, CO and PM, without the need to make any modification in the fuel supply system of the engine. Furthermore, as polyoxymethylene dimethyl ethers added into ordinary diesel cause the diesel to be diluted, accordingly, the contents of aromatic compounds and sulfides in the diesel fuel products are also reduced.
Synthesis of polyoxymethylene dimethyl ethers may be carried out by processing synthesis gas through a series of steps of methanol, formaldehyde, methylal, and polyformaldehyde etc. The verified coal reserves in China are about 714 billion tons, and developing coal-based methanol industry has huge resource advantages. However, the problem of excessive production capacity of methanol is particularly prominent in recent years. For example, the production capacity of methanol broke through 50 million tons in 2012, but the rate of equipment operation is merely about 50%. Thus the industrial chain of coal chemical industry is in an urgent need to be further extended. Therefore, developing a technologically advanced and economically rational industrial process for synthesizing polyoxymethylene dimethyl ethers based on methanol as upstream feedstock can not only provide a new technology to significantly improve diesel fuel product quality, but also improve the feedstock structure of diesel fuel production, so as to make it more suitable for the resource endowment of domestic fossil energy and enhance the strategic security of domestic supply of liquid fuel for engines.
In the aspect of synthesis of polyoxymethylene dimethyl ethers, a lot of work has been done at home and abroad, regarding research and development of methods for synthesizing polyoxymethylene dimethyl ether products where n=1-10 by using methanol, methylal, lower alcohol, aqueous formaldehyde solution, paraformaldehyde, etc. as feedstock in the presence of acidic catalysts.
In various kinds of feedstock route, more research has been done about the synthesis of polyoxymethylene dimethyl ethers from trioxane or paraformaldehyde together with methylal, including:
U.S. Patent Application US2007/0260094A1 discloses a preparation process of polyoxymethylene dimethyl ether using methylal and trioxane as feedstock in the presence of acidic catalyst. The water contained in the reaction mixture of methylal, trioxane and acidic catalyst should not exceed 1%. Polyoxymethylene dimethyl ether where n=3 and 4 in the reaction product is separated by rectification, and methylal, trioxane and polyoxymethylene dimethyl ethers with polymerization degree of n<3 and some of n>4 can be recycled.
A process of catalytic synthesis of polyoxymethylene dimethyl ethers with polymerization degree of methoxy groups at 2-10, by using methylal and trioxane as feedstock, in the presence of homogeneous or heterogeneous acidic catalysts such as liquid mineral acids, sulfonic acids, heteropolyacids, acidic ion-exchange resin, zeolite, etc., at the pressure of 1-20 bar and the reaction temperature of 50° C.-200° C. and under the condition of strictly limited water content introduced into the system, is disclosed in Chinese Patent Application CN101048357A of BASF Aktiengesellschaft. By optimization, polyoxymethylene dimethyl ethers with polymerization degree of methoxy groups at 3 and 4 can be separated by distillation through three towers.
Tianjin University discloses a process for synthesis of polyoxymethylene dimethyl ethers using methylal and trioxane as feedstock in Chinese Patent Application CN102432441A, which uses cation exchange resin as a catalyst in the fixed bed reactor, under the reaction condition of a reaction temperature of 80° C.-150° C., a reaction pressure of 0.6 MPa-4.0 MPa and a nitrogen atmosphere, mainly obtaining products with n at 3 or 4.
Furthermore, in recent years abroad, Jakob Burger etc. [i.e., Fuel 89 (2010) 3315-3319] synthesized DMMn using ion-exchange resin as a catalyst and methylal and trioxane as feedstock in a stirred-tank reactor in laboratory by intermittent operation, which focuses on studying the relationship between the reaction equilibrium composition and reaction temperature, feedstock mass ratio. In China, some colleges and universities such as East China University of Technology. Nanjing University, Lanzhou University of Technology, etc. are carrying out some basic and applied basic research in the aspect of chemical thermodynamics, catalyst screening and reaction process.
In conclusion, there has already been lots of research about preparing target product DMMn using methylal together with paraformaldehyde or trioxane as feedstock, the catalysts involved cover almost all the major types of acidic catalysts, but in the implementation process, no matter what kind of catalyst and reactor are used, the rate of chemical reaction is always very low, and the reaction is generally required to last for hours or even longer, it has become a major challenge which limits large-scale industrialization of this technology.